Vacuum bags are used in the low-pressure molding of various plastic, rubber, and resin bonded products, such as reinforced plastics, laminates, and bonded sandwich structures. The bag provides a means of applying pressure to the workpiece to be debulked or cured. Typically the bag materials are degraded during the process by the combination of heat, pressure, corrosive adhesives, oxidative atmospheres, etc., such that the bag either cannot be reused or can only be used for a limited number of cycles
Current practice in the manufacture o resin-bonded laminates for structural use is to pressure cure the product in the following steps:
1) A plurality of layers of resin impregnated fabrics or collimated tapes are applied to the surface of a tool to form a laminate. The layers or plies adhere to the tool and to each other by means of the tacky resin they contain. A typical resin is an epoxide composition.
2) A vacuum bag is positioned over the laminate and the perimeter of the bag is sealed to the tool. Air is drawn from the space between the vacuum bag and tool through a valve, built into the vacuum bag or tool, which is connected to a vacuum pump.
3) The tool, laminate and vacuum bag assembly is loaded into an oven or autoclave so that heat and pressure can be applied to effect consolidation and cure of the laminate. While the area between the tool and vacuum bag is maintained at reduced or atmospheric pressure, the pressure in the autoclave chamber is increased. The vacuum bag and tool are thus compressed together and the laminate between them is consolidated to produce a dense, strong article.
The vacuum bag approach provides a means of laminating and bonding at low pressures, i.e., 10 to 300 pounds per square inch. The system has many different uses and is able to accomodate workpieces of many shapes and sizes, limited only by the volume of the autoclave. In many cases, a single sided tool of modest construction, and hence cost, is all that is required. The tool only has to be impermeable and rigid at the process temperatures.
The prior art describes several different vacuum bags which act as a mobile pressure barrier, converting pneumatic or hydraulic pressure inside the autoclave to a mechanical pressure on the laminate. One basis for classifying different vacuum bags is whether they are intended for a single use or multiple uses.
A permanent bag is designed for multiple uses and is often an integral part of the tool. The permanent bags described in the prior art are based on elastomers which have high elongation and durability at the process temperature. U.S. Pat. No. 4,287,015 to Danner is typical, which describes a sheet of rubber which is attached and sealed to the tool and allowed to stretch over the laminate.
The permanent bags have had limited commercial success because of the short life of the materials used. The resin used during cure has a highly deleterious effect on many materials. For example, silicone rubber is one of the few elastomers capable of withstanding 350.degree. F., but silicone elastomers are rapidly degraded in contact with epoxide resins. The tear strength of silicone, which is not high in the original material, is significantly reduced after contact with curing epoxides. Permanent bags are often put out of service because of nicks and tears in the elastomer membrane which can grow rapidly under stress. Also, while silicone and fluorocarbon elastomers have useful properties at high temperatures, their high cost coupled with insufficient use cycles very often cannot justify their use. For the majority of vacuum bag uses, disposable materials are the most cost effective.
Another problem with permanent rubber pressure bags is that sheet rubber of sufficient thickness to withstand handling and demolding will not stretch into a curvature of small radius on the workpiece at the low pressures used. Still further, the mechanical properties of elastomers are anisotropic and depend upon morphology. Hence, a permanent bag often has difficulty in successfully producing even pressure on a laminate with deep contours.
A typical single use vacuum bag of the prior art is constructed as follows:
A thin vacuum bag film is attached to the tool and sealed to it by sealant putty tape. A breather layer is disposed below the film for removing gas which permeates through the film in order to maintain the reduced pressure between the film and tool. Other components used in the vacuum bag include release layers to facilitate separation of the laminate from the tool and the other layers of the vacuum bag. Perforated release layers are used to restrict and control the flow of excess resin which is bled from the laminate. Bleeder layers accept this excess resin. Caul plates are used to provide equal pressure and thus equal consolidation over certain areas of the laminate. Peel plies are used next to the laminate, and are removed in subsequent operations to provide a fresh surface for bonding.
The film for a single use vacuum bag has to maintain its integrity during heating to the cure temperature and subsequent cooling and act as a pressure-transmitting diaphragm. Early films were made of cellulose, polyvinyl acetate, or polyvinyl alcohol. Nylon films have substantially replaced these early materials. The two basic types, nylon 6 (polycaproamide) and nylon 6--6 (polyhexamethylene adipamide) are each used because of their high crystalline melt temperatures, 428.degree. F. for nylon 6 and 482.degree. F. for nylon 6--6. A- heat. stabilizer is added to ensure retention of the physical properties during exposures at high temperatures and oxidative atmospheres.
Nylon films have limited elasticity and have to be applied with care to produce an effective vacuum bag. The film is tucked and folded into a pleated arrangement by the insertion of vertical "ears" in the sealing putty tape. The application is important to the function of the bag and is provided in sufficient quantities according to the skill and know how of the person making the vacuum bag. During cure, the laminate consolidates and moves closer to the tool. The nylon films have limited elastic behavior and if bridged across a female corner or concave feature, one of two results is possible. If the bridging is large the nylon film will be stretched beyond its elastic limit (yield point). Because of the low tear strength and frequent faults in the nylon film, the bag will burst and loose pressure integrity. Alternatively, if the bridging is small so that the film can sustain the pressure without yielding, the bridging can prevent pressure acting upon that area of the laminate. A laminate beneath the bridging is not consolidated and the individual plies are not held together by a continuous matrix of resin. Thus, bridging of the bag can lead to a low-strength part which is not acceptable for its intended use. In an attempt to prevent bridging, pleats are applied in the nylon film prior to the application of pressure. However, considerable skill and labor is required to properly place the pleats and in many cases some bridging results even with the application of pleats.
The value of the composite laminate may be several thousand times the value of the vacuum bag used to form it. In the case of a major aircraft composite frame structure to be formed by vacuum bagging, a very large value item may be in jeopardy from a vacuum bag which fails.
The low tear strength of nylon films is also a disadvantage when the film is being handled and folded to produce the desired configuration of the vacuum bag. Often, when a nylon vacuum bag is tested before use by reducing the pressure below the bag and listening for leaks by the ear or at ultrasonic frequencies, small leaks are found large number of these leaks are found at or near folds. These small leaks have the potential to grow during heating and pressurization, causing a sub standard molding.
The extensibility of nylon film is a function of the moisture content of the film. Moisture content of the film is in equilibrium with the relative humidity of the air surrounding it. During the winter season, the relatively low humidity of the air, which causes the nylon film to become regid and brittle, results in more frequent bag failures. Formulations to modify nylons have met with little success. Plasticizing the nylon film lowers its temperature endurance, while adding humectants only delays the onset of embrittlement. Products of this type are available (e.g., U.S. Pat. No. 3,738.949) but have not replaced unmodified nylon films.
It is an object of the present invention to provide a material for a vacuum bag which can withstand the application of heat and pressure without failing.
Another object of the invention is to provide a vacuum bag material which readily conforms to the shape of the workpiece so as to evenly consolidate the workpiece.
Yet another object is to provide a vacuum bag which requires limited or no pleating in use.
Still another object is to provide a vacuum bag which is not dependent upon the moisture content of the environment for flexibility.
A further object is to provide a vacuum bag which stretches at fairly low applied pressures, i.e., about 5 to about 100 psi.
A still further object is to provide a vacuum bag which can be handled without damage and which will stretch across sharp corners and female contours of the workpiece or tool without bridging or tearing.
Another object of the invention is to provide a vacuum bag which saves cost in the materials used and extends the range of pressures which can be used.
Yet another object is to provide a vacuum bag which can withstand a specific applied pressure at a lower film thickness than prior art bags.
Yet another object is to provide a vacuum bag which can be reused several times with a large savings in bagging costs.
A further object is to provide a vacuum bag which can be removed from putty tape sealant without damage.
A still further object is to provide a vacuum bag whose physical properties are not significantly affected by the presence of epoxide or phenolic resins.